Phenolic Glycosides: Novel Markers for Identifying Smoke Taint in Grapes and Wine?

Many of those in Australia, parts of California, and other regions of the world that experience wildfires, grapes that are exposed to the smoke of these fires often possess what’s referred to as “smoke taint”: a highly undesirable smoky, ashy, or overall burned sensory characteristics that drive quality as well as price downward.  In order to test for smoke taint in wines, guaiacol and 4-methylguaiacol levels are often analyzed, though the downside to this method is that it’s not always reliable.  For example, oak barrels or chips will also impart guaiacol and 4-methylguaiacol into the wine, so simply finding these compounds in your wine doesn’t necessarily mean it suffers from smoke taint—it may have just been aged in an oak barrel!  Also, sometimes very little guaiacol is measured in the wine, when in fact later on smoke taint characteristics appeared as if from nowhere.

As a result of this inconsistency, there is a push to try and find better biochemical markers for smoke taint in wine, which will make diagnostic testing of grapes and/or wine so much faster and cost-effective, for if a reliable test for smoke taint

Photo By Jynto [CC0], via Wikimedia Commons

Photo By Jynto [CC0], via Wikimedia Commons

is developed, one could test the grapes right at harvest and not bother to make wine from those berries if, in fact, the smoke taint markers were present.

Past studies have indicated that after a grape takes up the smoke-derived volatile compounds, they undergo a chemical reaction resulting in what are known as “phenolic glycosides”, some of which have been identified (β-D-glucosyl-β-D-glucosides, β-D-glucopyranosides, pentosylglucosides, and rutinosides).  Basically what happens is that the grapes take up these smoke-derived volatiles and then convert them into more stable glycoside versions of the compounds.  Also, the greater the smoke exposure, the greater the levels of smoke-derived volatiles and stable glycosides.   These glycosides act as “precursors” to the smoke-derived volatile compounds, so if you have grapes that have been exposed to smoke and thus high levels of smoke-derived volatiles and their glycoside precursors, your wine is at risk for developing undesirable aromatic characteristics not only in the beginning of the winemaking process, but also through aging and storage.

The goal of the study today was to find another way to measure smoke taint in grapes and wine other than the current method of measuring guaiacol and 4-methyguaiacol.  Specifically, the study aimed to use smoke-derived stable phenolic glycosides as smoke taint markers, in hopes to find a quick and reliable method for winemakers to measure potential smoke contamination in grapes.

Methods

Chardonnay and Shiraz grapes that were exposed to the smoke from bushfires in Victoria, Australia in 2009 were harvested for analysis, as well as control grapes from other vineyards around Australia that had not been exposed to smoke.

Chardonnay and Cabernet Sauvignon wines that were previously created from grapes that were exposed to smoke were also analyzed.

Phenolic glycosides were analyzed using HPLC-MS/MS techniques, and guaiacol was analyzed using GC-MS techniques.

Photo from the holdings of the National Archives and Records Administration, cataloged under the ARC Identifier (National Archives Identifier) 196492.

Photo from the holdings of the National Archives and Records Administration, cataloged under the ARC Identifier (National Archives Identifier) 196492.

Results

  • The techniques used for both the red and white grapes/wine seemed efficient and accurate in analyzing phenolic glycosides, and was also repeatable (a good quality to have!).
  • Phenolic glycoside levels were apparently higher in smoke-exposed grapes and wine than levels of guaiacol.
  • There was a relatively wide range of glycoside levels in each grape variety, which may be due to the position of the grape on the vine, and how exposed to the smoke it was while still on the vine (this would need to be tested).
  • The most abundant glycoside in all varieties/varietals was Syringol gentiobioside, 5, then 6, and finally the pentosylglucosides and rutinosides.
  • Individual glycoside levels were well correlated with total glycoside levels in grapes and wine.
  • Results indicated that smoked-derived volatile compounds, particularly their phenolic glycoside precursors, may be good markers for measuring smoke taint in grapes and wine.
  • Guaiacol levels in many of the known smoke-tainted grapes and wine were within the range of the control grapes/wine, which were not exposed to smoke.
    • Analysis indicated that measuring guaiacol alone was not sufficient to identify a wine as exposed to smoke or not, while it was clear from measuring phenolic glycosides which grapes/wines were exposed to smoke and which were not.
    • Using guaiacol as a marker, only 3 out of the 6 wines that were exposed to smoke taint were identified as such, while when using phenolic glycosides, the analysis was able to identify all 6 wines as being smoke-tainted.
  • The two best phenolic glycosides for identifying smoke taint in grapes and wines were Syringol gentiobioside 5 and Syringol gentiobioside 6.

Conclusions

The results of this study were significant in that it was determined that using phenolic glycosides, specifically Syringol gentiobioside 5 and Syringol gentiobioside 6 as markers for smoke taint in wine is much more effective and accurate than measuring guaiacol levels.  According to the authors of this study, these findings indicate a vast improvement over current smoke taint analysis measurement techniques, and using phenolic glycosides may in fact be more accurate as well as time efficient.

It is important to note that this study was just a pilot study: meaning the sample sizes were relatively low.  It would be very interesting to see this study done on a large scale, incorporating many different grape varieties both exposed and not

Photo By U.S. Department of Agriculture (Flickr: 2012020-FS-UNK-0018) [CC-BY-2.0 (http://creativecommons.org/licenses/by/2.0) or Public domain], via Wikimedia Commons

Photo By U.S. Department of Agriculture (Flickr: 2012020-FS-UNK-0018) [CC-BY-2.0 (http://creativecommons.org/licenses/by/2.0) or Public domain], via Wikimedia Commons

exposed to smoke, and from different areas of the world.  According to the authors of this study, they are currently performing such a study, so hopefully once that is finished and the results are published, I can bring you more convincing (or not!) results.   These future studies will also be examining improving the reliability of the analysis, as well as specific recommendations for analytical methods that one could use in the laboratory to test grapes for smoke taint exposure prior to going through the winemaking process.

What do you all think about this study as well as this new technique?  Do you have any experience in this field?  Do you have any other ideas on how to more accurately test for smoke taint at any early stage so that one does not waste the time and resources on creating the wine before one realizes it’s contaminated?  Please feel free to leave any comments you have!

Source: Hayasaka, Y., Parker, M., Baldock, G.A., Pardon, K.H., Black, C.A., Jeffery, D.W., and Herderich, M.J. 2013. Assessing the impact of smoke exposure in grapes: Development and validation of a HPLC-MS/MS method for the quantitative analysis of smoke-derived phenolic glycosides in grapes and wine. Journal of Agricultural and Food Chemistry 61: 25-33.

3 comments for “Phenolic Glycosides: Novel Markers for Identifying Smoke Taint in Grapes and Wine?

  1. Michael Richmond
    August 8, 2013 at 8:02 pm

    The word taint implies a negative experience. I think the exposee of “smoke taint” may be doing more harm than good in the routine experience of most wine consumers. I’ve had wines rejected by customers who detected the faint smokiness of the oak barrels which cost me $1000 each exclaiming in their new knowledge and phony outrage,” SMOKE TAINT”! Two years ago that same taster would have reveled in the faint smokiness in the background of the same wine. It is not unreasonable to think of any discernible influence from atmospheric smoke as an artifact of the vintage. How does this differ from the “taint” of eucalyptol, brettanomycese, bay trees, the garrigue of southern France, the discernible variance among barrels from different coopers,etc.Too much of any of these elements can yield a disharmonious experience.Wine is the fermented product of the extract of grapes and barrels along with the unique combination of environmental influences. A wine taster has come of age when he or she can allow their palate to judge the pleasure of a wine and not allow there pleasure to be highjacked by sensationalistic journalism or corrupted by some new awareness. My personal experience with most of these “taints” is that with the passage of only a modest amount of time that element unfortunately becomes so integrated as to be only vaguely if at all discernible. The adventure of wine thrives on the edges of the predictable. I’d rather make a fascinating wine than a classic. This is much ado about nothing, creating a problem where none exists.

    • Becca
      August 9, 2013 at 7:22 am

      You make good points, Michael! Thank you for sharing your opinions and experiences!

      There certainly is this strong desire among many to create the “perfect” wine that is the exact same year to year, which in some ways does a disservice to the grape and everything it’s been through during the growing season. I, like you, like the idea of a “fascinating wine” instead of a classic, or in other words, the same thing over and over and over again. Maybe the grapes were exposed to a lot of smoke that year, or maybe they were located close to a bunch of eucalyptus trees: let the wine speak for the grapes!

      At the same time, I understand that wineries need to stay afloat, and if they are able to find a way to make what they think is the “perfect wine” every year, then this sort of technology would be great for them.

      I’d love it if more people would chime in on this: I think it’s a fascinating topic and certainly a fun one for discussion.

      Do we have any advocates of the other side out there? Please share your points!

    • WineKnurd
      August 11, 2013 at 1:23 pm

      Michael,

      I don’t disagree with what you are saying, but to play devil’s advocate perhaps your tasters either a) do not like smokey notes in a wine, b) are more sensitive to smokey compounds than yourself, or c) never liked your smokey wines but never knew how to express it without sounding like an idiot refuting the winemaker, but now can claim “smoke taint”.

      It would be easy enough to tell the customers that your grapes were not subject to the smoke from a wildfire before tasting to subjectively counter the effects “bad journalism”, and that the smokey notes are a positive complexity from your oak treatment. Smoke taint is generally vintage / harvest specific, as the previous year’s fire are no longer burning.

      If they still do not like it, well you can’t please everyone. In any event, make the wine you want to make and those that like it will buy it.

What do you think about this topic?